The proposed study will examine the ways that collective efficacy may modify the association between neighborhood concentrated disadvantage and youth violent outcomes in Medellin, Colombia, and the United States. It will use a cross-sectional subsample of 1,500 children and adolescents, aged 12-18, from the Project on Human Development in Chicago Neighborhoods (PHDCN), a 6-year prospective study of determinants of youth externalizing problem behaviors, as welt as a subsample of 916 adolescents aged 12-24 from a cross-sectional household survey of perpetrators, victims and witnesses of violence in Medellin, Colombia, to test the hypothesis that collective efficacy modifies the direct association between disadvantage and youth violence in a non-linear fashion. Specifically, I hypothesize that collective efficacy will have the strongest protective impact on violence in contexts of higher disadvantage. Collective efficacy will cease to be a significant protective factor against violence in neighborhoods of extreme disadvantage. The study will also compare the associations between the sub-dimensions of collective efficacy (cohesion and social control) and violence in two distinct national contexts.